Method for treating zinc cadmium lead and tin dross

ABSTRACT

Drossing fluxes for zinc, lead or tin comprise a salt of a sulphur or phosphorus acid which contains hydrogen in the anion, and either an alkali metal chloride or an alkaline earth metal chloride.

United States Patent Boddey et al. Feb. 19, 1974 METHOD FOR TREATING ZINC [56] References Cited CAPMIUM LEAD AND N "ROSS FOREIGN PATENTS OR APPLICATIONS [75] inventors: Ronald Frederick Boddey; lan 18,601,507 6/1960 Great Britain 75/78 James, both of Nechells, England [731 Assignee: Tosecointernafiofial [15mm y Bizot Birmingham, England Assistant Examiner-M. J. Andrews Attorney, Agent, or Firm-Wolfe, Hubbard, Leydig, [22] Filed: May 26, 1971 v i and Osann [21] Appl. No.: 147,230

57 ABSTRACT 0] e g Appl ion Pri ri y Data Dressing fluxes for zinc, lead or tin comprise a salt of June 9, 1970 Great Britain 27885/70 a sulphur or phosphorus acid which contains hydrogen in the anion, and either an alkali metal chloride or an [52] U.S. Cl 75/71, 75/77, 75/85, lin ar h me l hlorid I 7 75/86, 75/93 R, 75/24 [51] Int. Cl CZZb 17/00 [58] Field of Search 75/24, 71, 77-79, 7 Claim" Dram 75/85-88, 93 R, 93 AB, 93 AC, 93 AD, 93 DA, 93 G, 94; 148/23; 164/55-56 This invention relates to fiuxing agents for metals and more particularly to fluxing agents for molten zinc, cadmium, lead and tin and alloys of any of these.

The metals zinc, cadmium, lead and tin readily combine with oxygen so that, on exposure to the atmosphere the surface of the molten metal will become coated with a film of the oxide of the metal. These oxide films are wetted by the molten metal and, as a result of turbulence commonly occurring in the handling of the molten metal, a dross is formed on the surface of the metal which consists essentially of intimately associated metal and oxide. It is necessary in order to secure high quality cast products from'the molten metal that the dross should be removed from the molten =metal but this in fact is a difficult operation to effect satisfactorily, is time consuming, and represents, because of the high metal content of the dross, a substantial economic loss. 7

It is accordingly customary to add to the metal bath a fiuxing agent usually referred to as a drossing flux. Basically these fluxing agents have some solvent action on, or react with, the metal oxides and thus release from the dross a proportion of the entrapped metal. The action of the fluxing agent is often exothermic and so contributes to the release of the metal in molten condition so that it can flow back to the main body of the molten metal. The residue left on the surface of the molten metal is then, optimally, a dry powder of very low metal content.

The most commonly used drossing fluxes for zinc, cadmium, lead, tin and their alloys, rely on reactions involving the formation of hydrochloric acid and oxychlorides, and it is principally by this means that successful metal/oxide separation is achieved. Suitable fluxing materials are for example ammonium .chloride, zinc ammonium chloride and zinc chloride and whilst these are highly effective, they suffer from the serious disadvantage of producing copious fume at operating temperature. This fume is obnoxious and sometimes injurious to health.

Since the fume evolved usually contains a proportion of the fluxing agent itself an excessive generation of fume usually indicates an inefficient utilisation of the fluxing agent.

It is an object of the present invention to provide a new flux composition for theaforesaid metals, and a new process of treating drosses of such metals by means of such flux compositions, which avoids or minimises the generation 'of fume and is thus of greater efficiency.

According to the present invention there is provided a method for the treatment of drosses on molten metal, the metal being zinc, cadmium, lead, tin or an alloy of any of these, which comprises adding to the dross a flux composition comprising a salt of a sulphur acid or a phosphorus acid which contains hydrogen in the anion, and an alkali metal chloride or alkaline earth metal chloride. Preferably the cation of the said. salt is an alkali metal or (where the acid contains three or more hydrogens) an alkaline earth metal. Salts'containing hydrogen in the anion are generally called acid salts and are so referred to herein.

A preferred flux composition comprises an alkali metal bisulphate or an alkaline earth metal hydrogen 2 phosphate and an alkali metal chloride. Whilst the alkali metal may be sodium, potassium or any other alkali metal it is usually preferable on grounds of economy alone, to use the sodium salts. Where an alkaline earth' metal salt is used the cation is preferably calcium. The salts of the flux composition of this invention react together by metathesis to yield an alkali metal or alkaline earth metal salt of the acid, free from hydrogen in the anion, and hydrogen chloride, the latter of which acts as the effective fiuxing agent. It is found that the hydrogen chloride is generated by the aforesaid reactions at rates comparable to that at which it is used up in the fiuxing action, so that there is little or no tendency to generate excessive fume.

Preferred admixtures according to the present invention are -(a) sodium bisulphate and sodium chloride, and (b) one or more calcium hydrogen phosphates, for instance in the form of calcium superphosphate (a mixture containing calcium hydrogen phosphates) and sodium chloride. l

The weight ratio of said acid salt to chloride salt is preferably between 0.5:1 and 9:1, and these materials may form the whole or part of the flux composition. Where they form only part of the composition the remaining ingredients may be conventional fluxing material, such as ammonium chloride, zinc ammonium chloride or zinc chloride or may be relatively inert carbonaceous materials such as coke, wood flour or silicious materials in a finely divided state. The inert materials act as reaction regulators or inspissators.

If desired there may also be included in the flux com- ;position ingredients which, at the temperature of the molten metal, will promote exothermic reaction between the flux and the molten metal. Such materials are, for example, fuels, oxidising agents and fluorides as commonly used in exothermic compositions.

Since the essential ingredients of the flux compositions of the present invention (acid salts of sulphur and phosphorus acids, and alkali metal or alkaline earth metal chloride) may be inter-reactive there may be some tendency for such interreaction to take place after the mixing of the flux components but before the flux is put to use. To guard against this one or both of the active substances may be in the form of granules or particles coated with a protective layer, e.g., of a filmforming material such as a drying oil, fuel oil, lubricating oil, stearic acid or other high molecular weight fatty acid, or a wax or natural or synthetic resin. Methods of encapsulating particles in a protective sheath are well known per se. If the technique of preventing such interreaction just described is not employed it may be preferable to add the acid salt and chloride separately to the dross at the time of use, and to mix them by rabbling them into the dross.

The invention includes not only the method defined above'but also new fluxing compositions which comprise an acid salt of a sulphur acid or a phosphorus acid containing hydrogen in the anion and an alkali or alkaline earth metal chloride together with one or more further fiuxing agents and/or one or more inert solid diluents. The invention also includes new fluxing compositions in which at least one of the active constituents, i.e., the acid salt or the chloride, is provided with a protective coating to inhibit or reduce the tendency of the two sets of particles to chemically interact.

The following examples will serve to illustrate the invention:

EXAMPLE I Sodium bisulphate Sodium chloride 70 parts by weight 30 parts by weight EXAMPLE 2 Closely similar results were obtained using a fluxing composition comprising:

70 parts by weight Calcium superphosphate 30 parts by weight Sodium chloride in a similar proportion relative to the metal.

EXAMPLES 3 AND 4 Closely similar results were obtained using a mixture of:

Sodium bisulphate Sodium chloride 80 parts by weight 20 parts by weight and using a mixture of:

90 parts by weight Calcium superphosphate 10 parts by weight Sodium chloride EXAMPLES 5 AND 6 Further effective fluxing mixtures are:

Sodium bisulphate 40 parts by weight 20 parts by weight 15 parts by weight 25 parts by weight Sodium chloride Granulated chamotte Magnesium chloride and 45 parts by weight 15 parts by weight 20 parts by weight 15 parts by weight 5 parts by weight Calcium superphosphate Sodium chloride Granulated coke Magnesium chloride Sodium Nitrate We claim as our invention:

1. A method for treating the dross layer containing material which collects on the exposed surface of a molten metal selected from the group consisting of zinc, cadmium, lead, tin and alloys thereof which comprises adding to the dross a flux composition consisting essentially of (a) at least one salt of sulphur acids containing hydrogen in the anion and (b) at least one chloride selected from the group consisting of alkali metals chlorides and alkaline earth metal chlorides, the weight ratio of said salt to said chloride being in the range of 0.5:1 to 9:1, in an amount sufficient to release at least a portion of the metal from the dross layer.

2. The method of claim 1 wherein the cation of said salt is a member selected from the group consisting of alkali metals and alkaline earth metals.

3. The method of claim 1 wherein the cation of said salt is sodium.

4. The method of claim 1 wherein the cation of said salt is calcium.

5. The method of claim 1 wherein the cation of the chloride is sodium.

6. The method of claim 1 wherein the salt is sodium bisulphate and the chloride is sodium chloride.

7. The method of claim 1 wherein at least one of said salt and said chloride consists of particles having a protective coating thereon of a material selected from the group consisting of oils, fatty acids, waxes and natural and synthetic resins. 

2. The method of claim 1 wherein the cation of said salt is a member selected from the group consisting of alkali metals and alkaline earth metals.
 3. The method of claim 1 wherein the cation of said salt is sodium.
 4. The method of claim 1 wherein the cation of said salt is calcium.
 5. The method of claim 1 wherein the cation of the chloride is sodium.
 6. The method of claim 1 wherein the salt is sodium bisulphate and the chloride is sodium chloride.
 7. The method of claim 1 wherein at least one of said salt and said chloride consists of particles having a protective coating thereon of a material selected from the group consisting of oils, fatty acids, waxes and natural and synthetic resins. 